Dysregulation of diacylglycerol acyltransferase 1 may play a role in the development of obesity. Upon differentiation of mouse 3T3-L1 cells into mature adipocytes, a 90 fold increase in diacylglycerol acyltransferase 1 levels is observed. However, forced overexpression of diacylglycerol acyltransferase 1 in mature adipocytes results in only a 2 fold increase in diacylglycerol acyltransferase 1 levels. This leads to an increase in cellular triglyceride synthesis without a concomitant increase in triglyceride lipolysis, suggesting that manipulation of the steady state level of diacylglycerol acyltransferase 1 may offer a potential means to treat obesity (Yu et al., J. Biol. Chem., 277, 50876-50884 (2002)).
In a random Turkish population, five polymorphisms in the human diacylglycerol acyltransferase 1 promoter and 5′ non-coding sequence have been identified. One common variant, C79T, revealed reduced promoter activity for the 79T allele and is associated with a lower body mass index, higher plasma cholesterol HDL levels, and lower diastolic blood pressure in Turkish women (Ludwig et al., Clin. Genet., 62, 68-73 (2002)).
Diacylglycerol acyltransferase 1 knockout mice exhibit interesting phenotypes which indicate inhibition of diacylglycerol acyltransferase as a potential treatment for obesity and obesity-associated insulin resistance. Mice lacking diacylglycerol acyltransferase 1 are viable and can still synthesize triglycerides through other biological routes. However the mice are lean and resistant to diet-induce obesity (Smith et al., Nat. Genet., 25, 87-90 (2000)), have decreased levels of tissue triglycerides, and increased sensitivity to insulin and leptin (Chen et al., J. Clin. Invest., 109, 1049-1055 (2002)). Small molecule approaches to modulating the synthesis of diacylglycerol acyltransferase 1 are ineffective. (Tabata et al., Phytochemistry, 46, 683-687 (1997); Tomoda et al., J. Antibiot. (Tokyo), 52, 689-694 (1999)).
Diacylglycerol transferase 2 possesses diacylglycerol transferase activity that utilizes a broad range of long chain fatty acyl-CoA substrates (Cases et al., J. Biol. Chem., 276, 38870-38876 (2001); Lardizabal et al., J. Biol. Chem., 276, 38862-38869 (2001)). Diacylglycerol transferase 2 is a member of a family of genes whose sequences are unrelated to diacylglycerol acyltransferase 1. (Cases et al., J. Biol. Chem., 276, 38870-38876 (2001)).
Diacylglycerol transferase 2 mRNA is preferentially upregulated by insulin treatment, as shown by in vitro assays measuring the diacylglycerol activity from the membrane fraction of cultured mouse adipocytes. In fasting mice, diacylglycerol transferase 2 expression is greatly reduced, and dramatically increases upon refeeding. The expression patterns of two enzymes that participate in fatty acid synthesis, acetyl-CoA carboxylase and fatty acid synthase, respond to fasting and refeeding in a similar fashion. These results, combined with the observation that diacylglycerol transferase 2 is abundantly expressed in liver, suggest that diacylglycerol transferase 2 is tightly linked to the endogenous fatty acid synthesis pathway (Meegalla et al., Biochem. Biophys. Res. Commun., 298, 317-323 (2002)).
Studies of mice harboring a disruption in the diacylglycerol acyltransferase 1 gene provide evidence that diacylglycerol acyltransferase 2 contributes to triglyceride synthesis. Levels of diacylglycerol transferase 2 mRNA expression are similar in intestinal segments from both wild type and diacylglycerol transferase 1-deficient mice. Using magnesium chloride to distinguish between diacylglycerol transferase 1 and 2 activity, Buhman, et al. observed that, in diacylglycerol transferase 1-deficient mice, diacylglycerol transferase activity is reduced to 50% in the proximal intestine and to 10-15% in the distal intestine (Buhman et al., J. Biol. Chem., 277, 25474-25479 (2002)).
Additionally, diacylglycerol transferase 2 mRNA levels are not up-regulated in the liver or adipose tissues of diacylglycerol transferase 1-deficient mice, even after weeks of high-fat diet. However, in ob/ob mice, which have a mutation in the leptin gene that results in obesity, diacylglycerol transferase 2 is more highly expressed than in wild type mice, suggesting that diacylglycerol transferase 2 may be partly responsible for the highly accumulated fat mass seen in these mice. Furthermore, the combined mutations of leptin and diacylglycerol transferase 1 leads to a three-fold elevation in diacylglycerol transferase 2 expression in white adipose tissue, compared to the levels in the same tissue from diacylglycerol transferase 1-deficient mice. These data suggest leptin normally downregulates diacylglycerol transferase 2 expression, and that the upregulation of diacylglycerol transferase 2 in white adipose tissue in these mice may provide an alternate pathway for the triglyceride synthesis that still occurs in leptin deficient/diacylglycerol transferase 1-deficient mice (Chen et al., J. Clin. Invest., 109, 1049-1055 (2002); Cases et al., J. Biol. Chem., 276, 38870-38876 (2001); Chen et al., J. Clin. Invest., 109, 175-181 (2002)).
Liver fibrosis is the excessive accumulation of extracellular matrix proteins that occurs in many types of chronic liver diseases. Advanced liver fibrosis results in complications such as cirrhosis, liver failure and portal hypertension; frequently requiring a liver transplant. Common causes of liver fibrosis include chronic hepatitis C infection, alcohol abuse and non-alcoholic steatohepatitis (NASH). NASH is characterized by obesity, type-2 diabetes mellitus, dislypidemia and, commonly, insulin resistance. Cellular mechanisms of liver fibrosis include the release of soluable factors from kupfer cells that will activate hepatic stellate cells (HSC) into fibrogenic myoblasts. Active HSC further secrete cytokines to perpetuate the active state. Following persistant injury, the active HSC produce large amounts of extracellular matrix proteins (ECM). Degradation of the ECM is prevented by the actions of cytokines, such as TIMPs. Currently, there is no standard therapy for liver fibrosis. As such, the recommended course of action is to remove the causative agent, which for NASH would include weight loss and specific treatments for metabolic syndrome. (See e.g., Battler, R. and Brenner, D. A., J. Clin. Invest. 115:209-218 (2005) and supplement; Elsharkawy, A. M., Oakley, F. and Mann, D. A., Apoptosis v. 10, n. 4, 927-939 (2005); and Rockey, D. C. Clinincal Gastroenterology and Hepatology 3:95-107 (2005)).
There is a recognized need in the art for a treatment for liver fibrosis.